The present invention relates to a turbocharged two-stroke cycle internal combustion engine which can operate in a four-stroke mode under idling or starting conditions in order to eliminate the need for an externally driven air pump.
A two-stroke cycle internal combustion engine, in which a cycle consists of a power stroke and a compression stroke each occurring once during each revolution of the crankshaft, produces more power at a given rate of engine speed than a four-stroke cycle engine. This is because in a four-stroke cycle, a power stroke only occurs once very other revolution of the crankshaft. Under a condition of reduced load, however, the four-stroke cycle is more efficient at a given engine speed since less fuel is consumed. It would be advantageous, therefore, for a two-stroke cycle engine to have the capability of shifting to a four-stroke cycle mode in these situations.
In a two-stroke cycle, the exhaust valve must open once during each revolution of the crankshaft toward the end of the power stroke. An inlet valve or port must also open at around the same time to charge the cylinder with fresh air or an air/fuel mixture and displace the burned exhaust gases. A four-stroke cycle, on the other hand, consists of intake, compression, power, and exhaust strokes. The exhaust valve must open on every other revolution of the crankshaft during the exhaust stroke. The intake valve must also open on every other crankshaft revolution during the intake stroke.
Numerous methods exist in the prior art for enabling an engine to operate in both two and four stroke cycle modes. For example, if crankcase scavenging (explained below) is utilized for two-stroke operation, another port connecting the crankcase with the cylinder must open during the power stroke while the intake valve used in four-stroke operation must remain permanently closed. Or, if no crankcase scavenging is used, a supercharger or air pump forces a fresh charge through a port in the wall of the cylinder which is uncovered by the piston when it reaches bottom dead center, the four-stroke intake valve again being permanently closed. Alternatively, the four-stroke intake valve port may be used by the air pump during two-stroke operation. With any of these engine types, in order to go from two-stroke to four-stroke operation, means must be employed to cause the inlet and exhaust valves to open at appropriate times once every other crankshaft revolution. U.S. Pat. Nos. 2,178,152 and 1,792,028 disclose methods employing two sets of cams with mechanical means for switching between them. U.S. Pat. No. 4,392,459 discloses a method utilizing a computer to control the operation of the valves. All of these methods are somewhat complex which decreases the reliability and increases the cost of the engine. It would be advantageous if the operation of an engine could be shifted from four-stroke to two-stroke or vice versa simply by changing the speed of the camshaft which opens and closes the exhaust valve.
Two-stroke cycle engines may or may not utilize crankcase scavenging. Crankcase scavenging means that the crankcase is used to receive fresh charges of air/fuel mixture when the piston moves upward in the compression stroke. During the power stroke, a flow passage is established between the cylinder and the crankcase, and the inlet port of the crankcase is closed. Thus, the air/fuel mixture in the crankcase is forced into the cylinder by increased pressure and displaces the burned exhaust gases which exit through an exhaust port.
When crankcase scavenging is utilized, however, it is not possible to have an appreciable volume of oil in the crankcase as in an ordinary four-stroke engine as it would be incompatible with two-stroke operation. Instead, oil must be circulated through passageways in order to effect the lubrication function, or the oil is simply mixed with the fuel.
It is advantageous, therefore, for a two-stroke cycle engine to operate without crankcase scavenging. A two-stroke cycle engine without crankcase scavenging, however, must have some means for removing burned exhaust gases from the cylinder and recharging it with either air or an air/fuel mixture since the movement of the piston is not able to perform this function as with crankcase scavenging. One such means is an air pump driven externally. A more efficient means is a turbocharger in which is incorporated a turbine driven by the exhaust gases. Even turbocharged two-stroke engines, however, must have an externally driven air pump to supply air or an air/fuel mixture when the engine is started or idled due to the low speeds in such situations which do not produce sufficient exhaust pressure to drive the turbine. The requirement of an air pump in this application necessarily increases manufacturing costs. It would be advantageous, therefore, for a turbocharged two-stroke cycle engine to be able to be started or idled without the need for an externally driven air pump.